1. Field of the Invention
The present invention relates to a device of oscillating so-called terahertz wave.
2. Description of Related Art
Terahertz waves are electromagnetic waves generally having a frequency of 0.1 THz to 10 THz, and are expected for basic fields such as physical properties, electrospectroscopy, bioscience, chemistry and pharmaceutical science and for applied fields of atmospheric environmental assessment, security, material test, food test and communication.
As a device for oscillating terahertz waves, it has been developed a backward wave oscillator (BOW) and photomixing in several hundreds GHz band and free electron energy laser, p-Ge laser and quantum cascade laser (QCL) for 1 THz or more. These devices, however, have problems in the miniaturization and improvement of output power.
On the other hand, it has been recently developed a device for oscillating a wide-band terahertz wave with an optical switch or rectification using a femtosecond laser as a light source and applied for time domain spectroscopy (TDS) or the like.
According to “Basis and applications of terahertz wave” published by Kogyo Chosa Kai, 2005 and authored by Jun-ich NISHIZAWA, pages 105 to 115, stimulated Raman scattering (referred to as Polariton stimulated scattering) is caused by Polariton, which is hybrid wave of optically active transverse wave lattice vibration (TO Phonon) and terahertz wave in a crystal. It is thereby strongly generated parametric interactions of three kinds of waves, which are pump wave, idler wave and terahertz wave. As a result, when the pump wave exceeds a predetermined threshold value, idler and terahertz waves having coherency comparable with that of the pump wave are to be oscillated. Polariton stimulated scattering is observed in polar crystals such as LiNbO3, LiTaO3, GaP or the like. LiNbO3 has properties that (1) it is transparent in light wave region in a wide range (0.4 nm to 5.5 μm) and (2) resistive against optical damage, so that terahertz wave can be oscillated at a high output power.
Japanese Patent Publication No. H09-146131A discloses a device of oscillating terahertz wave using a z-plate of LiNbO3. The principle of oscillation of terahertz wave will be described referring to FIG. 1. FIG. 1 shows a main face 5a of a substrate 5 viewed from the above. The substrate 5 includes an incident face 5c of pump wave, an emitting face 5d of the pump wave, and side faces 5b and 5e. For example, a light source 1 irradiates the pump wave 3 onto the substrate and a light source 2 irradiates an idler wave 4. The pump wave 3 (frequency ω1), idler wave (frequency ω2) and polariton (terahertz wave: frequency ωT) satisfy law of conservation of energy (ω1=ω2+ωT) and law of conservation of momentum (noncollinear phase matching condition: k1=k2+kth), so that polariton stimulated scattering is observed. In this case, due to the scattering property, the frequencies of the idler wave 4 and terahertz wave 7 are decided depending on the angles α and θ of the pump wave 3 with respect to the optical axis.
According to the method, typically, the phase matching condition is satisfied when an angle α of wave vector k1 of the pump wave 3 and wave vector k2 of the idler wave is 0.5° to 1° and the terahertz wave 7 was oscillated (wavelength of 100 to 300 μm, frequency of 3 THz to 1 THz) at a high efficiency. Further, it is described that the terahertz wave is oscillated at an angle of 65 to 66° with respect to the idler wave.
Further, according to K. Kawase, M. Sato, T. Taniuchi, and H. Ito, (Appl. Phys. Lett.,) 68, PP. 2483, 1996, it is described that the intensity of the idler wave as well as that of the terahertz wave can be improved by constituting a resonator with the idler wave (terahertz wave parametric oscillation: TPO). In this case, by changing the angle α in a range of 1 to 2°, the terahertz wave of 0.97 THz to 2.2 THz can be oscillated. A y-plate of LiNbO3 crystal is used and silicon prism is pressed on a surface of the crystal so that (1) the terahertz wave can be drawn to the air at a high efficiency, (2) the fluctuation of the outgoing angle can be reduced (low dependency on wavelength) and (3) the terahertz wave having high directivity can be oscillated.
When such outer resonator is used, the spectrum line width of the oscillated idler wave becomes large (several hundreds GHz), so that it is difficult to generate terahertz wave having a frequency of 0.3 THz or lower generated by difference frequency. Because of this, the following two kinds of laser light sources have been used for generating the terahertz wave having a frequency of, for example, 0.3 THz or lower.
That is, according to Japanese Patent Publication No. 2002-72269A, an exciting laser light having a single frequency is irradiated and an idler wave having a single frequency is used for optical injection to generate terahertz wave having a high output power and whose spectrum line width can be reduced. A silicon prism is, however, used for drawing the terahertz wave.
According to “Cherenkov-type phase-matched widely tunable THz-Wave generation using lithium niobate waveguide” (the 56'th Applied Physics Related Association Conference, Proceedings, 2009 spring, Tsukuba University, 30p-P1-3), a slab optical waveguide of a y-plate or a z-plate of lithium niobate is applied to wide-band tuning of terahertz wave light source. It will be described referring to FIG. 2. A pump wave 3 and idler wave 4 are made incident onto an incident face 9c of an optical waveguide of a substrate 9, which is composed of a y-plate made of lithium niobate doped with magnesium oxide. 9a represents an upper face, 9f represents a bottom face and 9e represents a side face. Although it is described that the thickness of the slab optical waveguide is 3.8 μm, the detail is not described. According to the method, (1) the thickness of the slab optical waveguide 9 is made 3.8 μm to confine the exciting light in the slab optical waveguide 9 to reduce the phase mismatch, (2) the thickness of the substrate 9 is lowered to oscillate the terahertz wave from the crystal surface and to avoid the absorption of the terahertz wave 7 into the crystal, and (3) a prism 6 is provided on the upper face 1a of the substrate 1 to draw the terahertz wave 7.